This invention relates to electrical converter apparatus for producing AC power at a desired frequency from a source of AC power of a different or varying frequency. In particular, the invention relates to thyristor phase controlled converters and cycloconverters.
In a naturally commutated cycloconverter, it is desirable to control the occurrence of firing pulses. This function has been provided by a control system that, for example, compares integrated values of a reference sine wave voltage and the cycloconverter output to determine appropriate firing points. These normally occurring firing pulses may, under some conditions affecting the system, produce firing pulses at instances that are not favorable for operation of the power switching devices, normally thyristors, in a manner for optimum system performance. Therefore, an end stop control means is provided with imposes limits on the occurrence of the firing pulses. The end stop control overrides the basic firing pulse control means in order to limit the output waveform so as to ensure reliable commutation.
An end stop is provided to inhibit firing before the voltage wave reaches a certain angle; this end stop is referred to as the rectification end stop. Another end stop is provided to force a firing pulse to occur, if it has not already naturally occurred, by the time the voltage wave reaches a further certain angle; this end stop is referred as the inversion end stop. Thus the end stop control establishes the earliest and latest instants at which each thyristor may be fired and is capable of naturally taking over, or commutating, the current from its predecessor.
Cycloconverters to which the invention generally relates are described, for example, in Pelly, Thyristor Phase-Controlled Converters and Cycloconverters, Wiley-Interscience, 1971, wherein Chapters Nine and Ten (pp. 229-277) discuss firing pulse control techniques, particularly including at pp. 259-271 description of end stop control techniques. Also, see Gyugyi et al., Static Power Frequency Changers, Wiley-Interscience, 1976, pp. 279-322 on control circuit principles and especially pp. 308-311 on end stop control.
In U.S. Pat. No. 3,818,315 by Gyugyi, Pelly and Stacey, June 18, 1974, for example, is described an end stop control means providing end stop pulses, developed independently of the reference voltage, to override the normally produced pulses when a reference voltage variation outside predetermined limits occurs. Such apparatus and other prior end stop control means have limits that are fixed at some preset phase angles relative to the source voltages. With these fixed settings, it has not been possible to obtain end stop limits that are reliably safe for commutation when the cycloconverter is subjected to load faults, without severely restricting the output under normal conditions. It was in order to achieve controllable end stop limits in accordance with system transients such as those arising from load faults that the present invention concerning a new type of end stop control came about.
In summary, the present invention provides, in combination with the basic elements of a thyristor phase controlled cycloconverter, a new end stop control means wherein the end stop limits are controllably determined for the inversion and rectification limits by independent sets of phase locked timing waves. These timing waves are derived in a way such that they can be modified in phase to constrain or broaden the permissable range of firing angle as required by system parameters including those occurring due to transients such as load faults. Each line to neutral voltage from the generator has associated with it means for producing the two timing waves for setting the rectification and inversion end stop limits for that phase.
Additional benefits provided by the present invention as compared to prior art systems include the controllability of end stop timing to ensure not only reliable communication of the cycloconverter thyristors but also assurance that commutation occurs within limits providing optimum system performance. In particular, the performance of systems in accordance with this invention can assure the output voltage of the cycloconverter is constrained within optimum limits in accordance with conditions such as source voltage, source frequency, and load current. By relating the controllable end stop means with load current sensing circuitry, fast acting limitation of fault currents is achieved. Included herein is fast excess current detector circuitry that is advantageous both in the present application to end stop controls as well as elsewhere.
By the use of the present invention, cycloconverters can be devised in which the multiple purposes of high reliability, quality and efficiency can be more readily achieved in an economical system that reduces generator rating requirements, such as by about 15%.